Rocket powered space vehicles used in space missions can generally be categorized into launch vehicles and payloads. Launch vehicles provide the primary thrust for launching and delivering a payload from the Earth's surface into orbit. Launch vehicles generally include one or more rockets arranged to fire at different times, or stages, as the launch vehicle travels from the Earth's surface into orbit. The different stages are fired sequentially, and typically include at least a first stage or booster stage and a second stage. The booster stage is designed to launch and deliver the payload a pre-determined distance above the Earth before exhaustion. Upon exhaustion of the booster stage, the second stage is fired to deliver the payload the remainder of the distance into a desired orbit.
Rockets may use either a solid propellant or a liquid propellant. In the case of a liquid propellant system, typical related components include at least one propellant tank, a combustion chamber and a nozzle for accelerating/discharging the combustion product. Liquid propellant rockets generally use a liquid fuel and liquid oxidizer, which are stored in separate propellant tanks and brought into contact in the combustion chamber of an engine to provide thrust. Such liquid propellant rocket systems have gained favor for many space applications because of their performance, economics, safety, fluttering capabilities and inherent flexible missions designs.
For a booster stage that uses only a single liquid propellant rocket, it is known to actively monitor propellant levels in the two propellant tanks (the fuel and oxidizer being the two “propellants”), and to dynamically adjust the flow of the propellants such that they are depleted at virtually the same time. This maximizes the amount of propellant burned prior to engine shutdown and jettison of the rocket. Such apparatus, methods and systems have been disclosed in commonly owned U.S. patent application Ser. No. 10/052,126, filed Jan. 17, 2002 titled “PROPELLANT UTILIZATION SYSTEM”, now U.S. Pat. No. 6,631,314, the contents of which are incorporated herein by reference in their entirety.
Simultaneously depleting liquid propellants becomes more complicated when the booster stage comprises more than one rocket. In a multi-rocket system, each of the rockets has a potential to run out of liquid propellant at different times even though the ratio of fuel and oxidizer may be identical. This is primarily due to the rockets burning propellant at different rates. While each rocket is designed and calibrated to burn a specified amount of propellant each second, there can be noticeable variations due to actual operating conditions, acceptance test inaccuracies and other unforeseen occurrences. As a consequence, there exists a risk that the rockets will not run out of propellant at the same time. This is disadvantageous because once one of the rockets has gone below a threshold propellant level, the flight computer will shut down all similar stage rockets and jettison them even though propellant remains in the other rockets. Thus, the first rocket to fall below the predetermined specified threshold will have burned essentially all of its available propellant, but the other remaining rockets will have more than the threshold level of propellant remaining. The unused propellant ends up being “dead weight” that was carried aloft unnecessarily by the launch vehicle, affecting the lift capability of the system.